This invention relates to optical displacement sensors and has application in pressure sensors in which pressure is indicated by the displacement of a diaphragm.
One known kind of optical displacement sensor comprises an optical fibre the tip of which is positioned to direct light transmitted along the fibre onto a reflector the displacement of which is to be measured. The reflector may comprise a diaphragm or membrane having a reflecting surface and forming part of a pressurised enclosure. The optical system is arranged so that the change in the amount of light reflected back is a measure of the displacement of the reflector. The reflected light is measured and its value indicated to give a measure of displacement or pressure. Desirably the optical system is arranged so that the change in reflected light is linearly related to displacement.
While a sensor of the kind described above is accurate immediately after calibration it will be appreciated that the magnitude of the detected signal depends not only on the position of the reflector but also on any change in its reflectance that occurs after calibration. This quantity may be subject to aging and will affect the accuracy of the measurement.
It is an object of the invention to provide compensation for the effects of such aging.